The present invention relates to improvements in a method of manufacturing high-silicon steel, that is, Fexe2x80x94Si alloy steel called silicon steel or Fexe2x80x94Sixe2x80x94Al alloy steel called sendust which has a silicon content of 3 to 10 wt %. More specifically, the present invention relates to a manufacturing method for high-silicon steel that is very difficult to cold-roll into thin sheet, that is, for example, to a method of manufacturing rolled silicon steel sheet by fabricating a sintered body or melt ingot having an average crystal grain size is 300 xcexcm or smaller, and, by enhancing crystal grain boundary slip, cold-rolling the material as is, or to a manufacturing method for obtaining super-thin sendust sheet by fabricating a thin sheet-form sintered body made up of an iron-rich phase and a silicon-rich Fexe2x80x94Si solid solution phase, making cold rolling possible using the outstanding malleability of the iron-rich phase crystal grains, then, after cold rolling, causing aluminum to adhere to both sides of the thin sheet and performing heat treatment.
Currently, almost all of the rolled silicon steel sheet used widely in various applications such as iron cores in transformers and rotating machines, magnetic shielding materials, and electromagnets is manufacturing by repeatedly subjecting silicon steel ingots wherein the silicon content in the iron is 3 wt % or lower to the processes of heat treatment, hot rolling, and annealing.
It is known that permeability is maximized in silicon steel when the silicon content is around 6 wt %, but the rolling of silicon steel sheet wherein the silicon content is 3 wt % or greater in the iron has long been considered very difficult due to fractures occurring during rolling.
In general, the average crystal grain size in melt ingots of silicon steel having a silicon content of 3 wt % or greater in the iron is several mm or greater, and plastic transformation induced by rolling is primarily caused by slip transformation inside the crystal grains.
In cases where the silicon content exceeds 3 wt %, however, the crystal grains themselves become very hard or brittle, wherefore, in silicon steel melt ingots having an average crystal grain size of several mm or greater, cracks readily occur during rolling, irrespective of whether hot rolling or cold rolling is used, and rolling itself becomes virtually impossible.
This is why a method was proposed (K. Narita and M. Enokizono: IEEE. Trans. Magnetic. 14 (1978) 258) for adding magnetic impurities such as magnesium and nickel to make the average crystal grain size in melt ingots more minute. The problem with this method, however, is that these magnetic impurities reduce the magnetic properties of the silicon steel sheet, and so it has not come into wide use.
Another method has been proposed (Y. Takada, M. Abe, S. Masuda and J. Inagaki: J. Appl. Phys. 64 (1988) 5367), and implemented, for manufacturing silicon steel sheet having a desired composition, such as silicon steel sheet having a silicon content of 6.5 wt %, by impregnating the silicon using a CVD (chemical vapor deposition) method after rolling a melt ingot containing 3 wt % silicon in the iron in a conventional process. CVD requires many processes and involves high cost, however, wherefore the applications thereof are naturally limited.
In silicon steel, moreover, when the silicon content is increased, the electrical resistivity xcfx81 of the silicon steel also increases, which is useful in reducing eddy current loss, and is desirable in a soft magnetic material usable in high frequency areas, but this has not been made practical because of the problem of processability noted earlier.
On the other hand, the Fexe2x80x94Sixe2x80x94Al alloy (sendust) that excels as a soft magnetic material of high permeability is a steel material that ordinarily has a higher silicon content than the silicon steel sheet noted above, and the manufacture of thin sheet thereof has long been considered very difficult due to its great brittleness and hardness.
For this reason, a method has been proposed (H. H. Helms and E. Adams: J. Appl. Phys. 35 (1964) 3) for manufacturing thin sendust sheet of 0.35 mm or so thickness by first fabricating an ingot having lower iron content than the composition required for sendust, pulverizing this, adding iron powder to the pulverized powder to make the required composition, causing the iron powder to act as a binder, and then repeatedly rolling and heat-treating this material.
Methods which employ the powder metallurgy noted above suffer the problem of reduced magnetic properties due to inadequate diffusion of the added element, however, and so have not come into wide use.
For this reason, crystals of sendust having few flaws are fabricated, these are thinly machine-cut, and vapor-deposited by sputtering on a desired substrate to form a sendust thin sheet, the outstanding functioning whereof is used in VCR magnetic heads.
The situation, in other words, is that, conventionally, the volume of sendust thin sheet produced is very small, and the applications thereof are limited, due to the difficulty of mass-production which involves much time and effort.
An object of the present invention is to implement the rolling of silicon steel having a silicon content of 3 wt % or greater which has been conventionally considered impossible. To that end, another object of the present invention is to provide a manufacturing method for rolled silicon steel sheet, and rolled material, wherewith it is possible to easily make the average crystal grain size of the pre-rolled silicon steel sheet more minute, and wherewith the rolled material can be continuously and uniformly cold-rolled, as is, without repeatedly subjecting the silicon ingots to heat treatment, hot rolling, and annealing.
Another object of the present invention is to provide silicon steel wherewith it is possible, without impairing the magnetic properties proper to silicon steel, to sufficiently increase electrical resistivity xcfx81 and reduce eddy current loss.
Another object of the present invention is, in view of the current situation wherein laminated iron cores and the like cannot be configured due to the difficulty of manufacturing sendust thin sheet, to provide a method of manufacturing sendust thin sheet wherewith it is possible to manufacture sendust thin sheet by cold rolling and obtain sendust thin sheet having very outstanding magnetic properties.
The inventors reasoned that cold rolling would be possible, when rolling silicon steel sheet having a silicon content of 3 wt % or greater, by using a sintered body or thin melt sheet having an average crystal grain size made minute for the pre-rolled silicon steel material, and significantly improving grain boundary slip.
Similarly, the inventors reasoned that cold rolling would be made possible by using, for the pre-rolled silicon steel material, a sintered body wherein an iron-rich phase was caused to remain, and causing plastic transformation utilizing the crystal grain malleability exhibited by the iron-rich phase.
The inventors, as a result of various investigations made concerning rolling material for silicon steel exhibiting good cold-rolling characteristics, based on the ideas stated in the foregoing, focused on the average crystal grain size, and made sintered bodies and quick-cooled melts to fabricate silicon steel rolling material having an average crystal grain size of 300 xcexcm or less, made more minute than conventional silicon steel resulting from slow-cooling melts. They learned that rolling was possible by cold-rolling this, that the effectiveness of making the grain size minute is realized regardless of the silicon content, being particularly effective at and above 3 wt %, and that rolling can be done comparatively easily by making the sheet thickness of the rolling material 5 mm or less and the parallelism 0.5 mm or less.
Similarly, the inventors focused on the composition inside the crystal grains, fabricated sintered silicon steel sheet wherein an iron-rich phase with abundant malleability is caused to remain in a mixed phase having an iron-rich phase and a silicon-rich Fexe2x80x94Si solid solution phase, unlike the crystal grain of the phase where, with conventional slow-cooling of the melt, iron and silicon are caused to completely become a solid solution, and learned that rolling is possible by cold-rolling this.
The inventors also learned, in terms of the method for manufacturing a sintered body, that it is possible to fabricate a sintered body having the desired minute average crystal grain size by using powder metallurgy techniques to sinter gas-atomized powder or water-atomized powder having a prescribed composition. They further learned, in terms of the powder metallurgy techniques, that it is possible to adopt a method wherein, after molding by metal injection molding, green molding, or slip-cast molding wherein a slurry form of the powder is made to flow in, sintering is done at a prescribed temperature, or a method wherein fabrication is effected by a hot molding method such as hot pressing or plasma sintering, etc.
The inventors further learned, in terms of a method for fabricating thin melt sheet, that a method can be adopted wherewith, in order to make the average crystal grain size as minute as possible, the molten silicon steel is made to flow into a water-cooled casting mold having a thin casting thickness and rapidly cooled.
The inventors also learned, in terms of the composition of the rolling material, that by adding small amounts of Ti, Al, or V, etc., the average crystal grain size at the time of annealing, after rolling, is readily coarsened, that it is possible to completely make the iron-rich phase and silicon-rich phase a solid solution, and that thin rolled silicon steel sheet can thus be obtained that exhibits outstanding magnetic properties wherein the coercive force drops precipitously.
The inventors, having learned of the method of manufacturing rolled silicon steel sheet described in the foregoing, confirmed an increase in electrical resistivity xcfx81 associated with high silicon content. Thereupon, they conducted various investigations on additive elements with the object of finding a material wherewith eddy current loss could be further reduced, and learned that lanthanum is effective. After conducting further investigations, they learned that that, when silicon steel is fabricated with a sintering method, oxides of lanthanum are deposited in the crystal grain boundaries, and that, accordingly, their object can be realized.
The inventors also learned, in terms of a method for depositing the lanthanum oxides in the crystal grain boundaries, that, in addition to the sintering method noted above, that that can be achieved by taking a silicon steel ingot containing lanthanum and subjecting it either to repeated hot rolling or to repeated hot forging.
The inventors, having learned of the method for manufacturing rolled silicon steel sheet described in the foregoing, learned further that, by taking silicon steel sheet obtained by cold-rolling material formed of a sintered body or melt ingot, of silicon steel having a minute average crystal grain size, or silicon sheet obtained by cold rolling, using a sintered body wherein an iron-rich phase is made to remain, and utilizing the grain boundary malleability exhibited by that iron-rich phase, vapor-depositing aluminum under various conditions on both sides thereof and then performing heat treatment, the aluminum diffuses from the surface thereof into the interior, thereby yielding sendust thin sheet having outstanding magnetic properties wherein magnetic permeability is dramatically improved over that of silicon steel sheet.